The changes in sperm during maturation, capacitation, decapacitation, and fertilization are related to the properties of the membranes of the head. Data on enzymatic activity of the plasmalemma and outer acrosome membrane, as well as visualization of the glycoprotein layer in fine structure, demonstrate that head membranes are modified by physiological conditions. The present proposal has as its objective, analysis of both the enzymes and glycoprotein layer of the head membranes of cauda epididymal sperm. With this data as a base-line, modifications in the membranes by other regions of the epididymis, by seminal plasma, and by uterine fluid will be evaluated. The fertilization site on the sperm surface will also be analyzed for changes in enzymatic activity and surface coat components. The vitro capacitation of rabbit sperm will provide a population of cells which will enable the fine structural dissection of the effects of capacitation agents on the membranes of the head. Comparison of in vitro changes in the sperm surface with in vivo capacitated and fertilizing sperm may reveal the elements in female genital tract fluids that affect capacitation.